fix train_new_from_iterator in the case of byte-level tokenizers (#17549)

tests/models/gpt2/test_modeling_gpt2.py

@@ -166,6 +166,11 @@ class GPT2ModelTester:
             reorder_and_upcast_attn=reorder_and_upcast_attn,
         )
 
+    def get_pipeline_config(self):
+        config = self.get_config()
+        config.vocab_size = 300
+        return config
+
     def prepare_config_and_inputs_for_decoder(self):
         (
             config,